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Radiopharmaceuticals labelled with radionuclides of high radionuclide purity in nuclear medicine that emit gamma-rays and beta-rays have been used for diagnosis imaging of diseases and therapy in cancer treatments, respectively. Internal radiation therapy is performed using radiation doses to kill cancer cells and shrink tumors. A personalized medicine treatment that combines diagnosis and therapy has become important for a patient, in which information about the patient’s genes is used. A personalized medicine treatment is performed using radionuclide pairs appropriate for both diagnosis and therapy in the same patients, requiring us to develop a variety of radionuclides.

Recently, significant interest has arisen in ¹⁷⁷Lu, currently produced in reactors, owing to successful treatments in patients with neuroendocrine tumors, which appear everywhere in the body, using ¹⁷⁷Lu-labelled radiopharmaceuticals; this success encourages further study of ¹⁷⁷Lu production in accelerators toward widespread medical applications of ¹⁷⁷Lu. Indeed, the excitation functions of the ¹⁷⁶Yb(d,p)¹⁷⁷Yb®¹⁷⁷Lu and ¹⁷⁶Yb(d,n)¹⁷⁷Lu (hereafter ¹⁷⁶Yb(d,x)¹⁷⁷Lu) reaction have been measured using a natural Yb (^natYb) sample but not an enriched ¹⁷⁶Yb sample. Hence, radionuclide purity of ¹⁷⁷Lu radionuclide is low because of many impurity radionuclides of Lu arising from seven stable isotopes, ¹⁶⁸Yb-¹⁷⁶Yb, present in ^natYb. Even with the use of highly enriched samples, no radionuclide sample is 100% pure and all contain some impurity radionuclides resulting from the production process by nuclear reactions.

A method was developed to estimate the isotopic compositions of enriched ¹⁷⁶Yb samples required to produce ¹⁷⁷Lu with a high radionuclide purity using the ¹⁷⁶Yb(d,x)¹⁷⁷Lu reaction. First, the estimation was done by deriving excitation functions of individual reactions for producing all aforementioned impurity Lu radionuclides that were obtained by fitting the measured excitation functions of the ^natYb(d,x)Lu reactions with an appropriate function form (see Fig. 1). Second, the yields of all Lu radionuclides that were produced by irradiating enriched ¹⁷⁶Yb samples with several isotopic compositions of Yb present in enriched ¹⁷⁶Yb samples with deuteron beams were estimated. The ¹⁷⁷Lu radionuclide purity for the 99.90% and 97.60% enriched ¹⁷⁶Yb samples at 2.5 days after the end of irradiation was estimated to be high, that is, > 98% and > 99% at the deuteron energy of 20 and 15 MeV, respectively. The resultant radionuclide ¹⁷⁷Lu would play an important role in promoting its widespread use for a variety of therapeutic applications. This finding to utilize measured excitation functions of nuclear reactions with natural samples would play an important role in producing a variety of medical radionuclides with a high radionuclide purity.

(written by Yasuki Nagai on behalf of all authors)

Fig.1.

(Left) Measured and fitted excitation functions of the ¹⁷⁶Yb(d,2n)^176mLu reaction. (Middle) Measured excitation functions of the ¹⁷⁶Yb(d,p)^177g+mYb (filled squares), ¹⁷⁶Yb(d,n)¹⁷⁷Lu (filled triangle), and ¹⁷⁶Yb(d,x)¹⁷⁷Lu (cumulative. filled circles) reactions. The fitted excitation functions of the reactions are shown by the solid, dotted, and dashed lines.

(Right) Measured (filled circles) and fitted (solid, dotted, and dashed lines) excitation function of the ^natYb(d,x)^174g+mLu reaction.